Remove `len` from CombinedStreamingDataset (#19321) #518

Job	Run time
release-version	52s
build-packages / init	4s
build-packages / build-packages (lightning)	21s
build-packages / build-packages (app)	27s
build-packages / build-packages (fabric)	29s
build-packages / build-packages (pytorch)	30s
upload-packages	0s
pre-publish-packages	0s
signaling	0s
legacy-checkpoints / create-legacy-ckpts	1m 22s
waiting	0s
publish-packages	0s
legacy-checkpoints / add-ckpt-test	0s
	4m 5s

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